The present invention generally concerns the distributed generation of electricity by means of alternative sources such as photovoltaic cells, fuel cells, wind generators and the like, or via conventional sources such as autogenous units with small internal combustion engines for the local generation of electricity. More particularly, this invention pertains to methods of automatically interrupting a distributed power generating source when the power source is electrically disconnected from the power grid.
To deal with the problem of environmental pollution and the increasing domestic demand for electricity, the installation of distributed electricity generator units to power moderately-sized loads is currently encouraged for residential and commercial buildings or in the industrial sector. These generator units use alternative energy such as solar energy, by means of photovoltaic cells, or wind energy by means of wind generators. Typically an alternative source of this type that generates a direct current is combined with a power conditioning unit that includes an inverter. The inverter is connected in parallel to the public power grid so that a generic local load can be powered alternatively by the grid, by the inverter, or by both. When the power supplied by the alternative source is insufficient to power the load, it is supplied wholly or partly by power taken from the grid. Conversely, when the load absorbs less power than that available from the alternative source, or when the load is not powered, the power generated by the alternative source is fed into the grid.
FIG. 1 shows schematically a system comprising an alternative source 1, such as a battery of photovoltaic cells, connected to an inverter 3. The inverter 3 is connected to a node A to which a generic local load 5 is connected, schematically shown as a parallel R, L, C load circuit. The inverter is connected in parallel to the grid, schematically shown by an AC voltage source 7 connected to a grid line 8 for distribution of the electricity. A transformer 9 and a switch 11 separate the source 7 from the line 8.
The generic switch 11 is opened to isolate a portion of the grid from the grid power source 7 if, for example, maintenance work has to be carried out on that portion (e.g., line 8) of the grid. The switch 11 can also open automatically in the case of a short circuit for example.
When the switch 11 is opened, the alternative source 1 via the inverter 3 can continue to power both the load 5 and the portion of grid downstream of the switch 11. A situation of this type is potentially very dangerous. For example, having opened the switch 11, maintenance personnel may wish to work on line 8 to repair a fault. If the inverter 3 is supplying power, this can lead to the risk of electrocution.
A further serious drawback that can occur if the grid is temporarily disconnected from the load 5 is that the inverter 3 loses its phase synchronism with the grid voltage. This synchronism is normally maintained by means of a phase-locked loop (PLL), which keeps the frequency and the phase of the output of inverter 3 synchronized with the frequency and phase of the grid voltage. When the grid voltage is removed, the inverter 3 can begin to power the load with a voltage having a phase which changes in a non-controlled way. When the switch 11 is re-closed, the grid and inverter output voltages are no longer in phase.
It is therefore necessary to provide means to de-activate the inverter 3 when an interruption occurs in the connection between node A and the grid line 8. For this purpose, the power control and conditioning unit normally includes an over-voltage relay, a low limit voltage relay, a high limit frequency relay and a low limit frequency relay. It is known, in fact, that in the majority of operating conditions of a system of this type, when the grid is disconnected from the load a rapid variation occurs in the inverter output frequency and/or voltage. Therefore, in a very short time, in the order of tenths of a second, the output voltage or frequency of the inverter 3 can be sensed to activate one of the four protection relays, causing the inverter to switch off.
For a detailed discussion of how the four relays cut in, see U.S. Pat. No. 6,429,546, and also M. E. Ropp et al., Prevention of Islanding Grid-connected Photovoltaic Systems, in Progress in Photovoltaics: Research and Applications, 7, 1999, pages 39-59.
A situation exists, however, in which the four protection relays are not able to interrupt operation of the inverter. This occurs when, at the time the switch 11 opens, the load 5 is not absorbing power from the grid and its power factor is equal to 1. When this happens, opening of the switch 11, i.e. electrical separation of node A from the grid power source 7, does not cause any variation in the output frequency or voltage of the inverter 3. Accordingly, the inverter is not switched off. This condition is called islanding. Islanding is a safety hazard because the portion of the grid electrical line 8 that is connected to the inverter 3 remains powered.
In practice, islanding is not limited to situations when the power supplied by the grid to the local load has a zero value. On the contrary, the four protection relays described above will not operate under circumstances where the reactive power and active power absorbed by the grid are equal to zero.
Various systems have been studied to avoid this phenomenon of islanding, based on active methods or passive methods. An overview of the various methods currently used can be found in the article referred to by M. R. Ropp et al.
Further methods and devices to prevent the phenomenon of islanding are described in the following U.S. Pat. Nos. 6,801,442; 6,853,940; 6,429,546; 6,219,623; 6,172,889; 5,686,766; 5,493,485; 5,162,964; 4,878,208. The contents of these and other documents and patents referred to are entirely incorporated in the present description.
All of these methods are based on the idea of identifying an electrical parameter indicating a condition of islanding, or if necessary, actively inducing a variation in the inverter output. In general they are defined as active methods or passive methods of identifying the conditions of islanding according to whether a forced variation of the inverter output is induced or not. A typical active method for identifying the condition of islanding is based on impedance measurement. This method is based on the fact that by varying via a disturbance one of the three parameters of the current delivered by the inverter 3 (phase, magnitude or frequency), the variation will cause a consequent voltage variation at node A if the electrical grid (7, 8) is disconnected, i.e. if the system is in an islanding condition. If not, the disturbance is “absorbed” by the grid.
Whatever the method used to identify an islanding condition, the methods known in the prior art provide for de-activation of the inverter when the device detects a situation that is interpreted as a symptom of an islanding condition. Therefore, if in practice the system is not isolated, but the device has detected a false islanding condition, the inverter is inappropriately de-activated, with consequent unnecessary inconvenience for the user and loss of power generated by the alternative source.